The phenomenon of flat feet is very often due to misalignment of the ankle bone or “astragalus” and of the heel bone or “calcaneus”, causing a deformation in the foot arch.
In order to remedy this type of pathology, it is known that an implant or “endorthesis” can be inserted into the tarsal sinus. That implant then acts as a wedge whose function is to re-align the calcaneus and the astragalus. When the re-alignment of the calcaneus and of the astragalus is satisfactory, i.e., in general, about nine months after being fitted, the implant is removed by being extracted with forceps.
In some known implants, an outer casing made of polyethylene is in the form of a cylinder through the center of which a tunnel is bored into which a screw is inserted. The outer casing is provided with notches enabling it to expand under the effect of the advancing screw. By expanding, the implant opens the canal of the tarsal sinus so as to reposition the astragalus on the calcaneus.
The major drawback of that type of implant is that, in its expanded configuration, it does not have a shape that is complementary to the shape of the cavity of the tarsal sinus.
In order to mitigate that drawback, another surgical device has been developed that is made up of three components, namely:
a central body provided with an external thread, with a head for blocking movement in translation;
an expansion cone provided with an internal thread, suitable for co-operating with the external thread on the central body; and
an outer ring that can be expanded under the effect of the expansion cone moving in said outer ring while the central body is being tightened.
Such devices generally give good results. In particular, such devices can, by means of their particular shapes, be locked easily in the cavity of the tarsal sinus. However, those devices also suffer from non-negligible drawbacks.
In particular, the system of assembly by screw-fastening, which offers the advantage of being reversible and thus of facilitating early extraction of the implant by reducing its diameter, by loosening the expansion cone, suffers from the fact that the expansion can loosen spontaneously, while the device is implanted. In which case, the effectiveness of the implant is reduced, since said implant is no longer capable of acting as a wedge.